Such water traps are disclosed, for example, in U.S. Pat. No. 6,117,214 and United States patent application publication 2009/0084383.
In water traps used hitherto for anesthetic gases or for anaesthetic devices for measuring the respiratory gas of a patient drawn off by suction, either exclusively gas permeable and liquid impermeable PTFE-(Polytetrafluoroethylene)-membranes are used or filter elements are used in addition to a PTFE-membrane. The filter elements are intended to noticeably increase their gas flow resistance when coming in contact with water and to prevent water from penetrating.
Disadvantageous in the water traps of the first type without an additional filter element is that foreign substances, which are transported along with the sample gas stream, collect on the surface of the PTFE-membrane over time during the course of operation. These foreign substances reduce the hydrophobic effect of the PTFE-membranes and eventually lead to a wetting of the PTFE-membranes and to a through passage of liquids or condensate.
In water traps of the second type (for example, water traps of the Criticare Company), it is disadvantageous that the additional filter elements cause a significant pressure drop already in the dry state because of their structure. The pressure range available for taking patient gas measurements is thereby noticeably limited.
If, in the case of a fault, water passes through the membrane and reaches the filter elements, the pressure drop across these elements increases noticeably as a result of the fine pored structure. Nonetheless, the danger persists that liquids pass through these filter elements at corresponding pressure conditions, especially at pressure differentials of greater than 500 hPa.